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Chlorosis of Trees

Laurie Stepanek, "Simply Trees" from the Nebraska Forest Service

      Many gardeners are familiar with chlorosis, a nutrient deficiency that affects the production of chlorophyll, the green pigment found in leaves. The symptoms are fairly easy to recognize: pale green to yellow leaves with green veins. Sometimes the edges of the leaves or spaces between the veins die and turn brown.

      Chlorosis is common in pin oak, river birch, silver maple and red maple cultivars and hybrids such as Autumn Blaze, Autumn Flame and October Glory. Other susceptible trees include crabapple, amur maple, sweetgum, baldcypress and white pine.
      The root cause of chlorosis is a deficiency of certain micronutrients, usually iron or manganese, but solving the problem is complicated since several factors are involved. Chlorotic trees often occur in alkaline soils, which have a pH above 7. Alkalinity renders iron and manganese insoluble, which makes the micronutrients unavailable for root uptake. Simply adding iron or manganese to the soil, therefore, will not correct the problem. Urban areas often have alkaline soils because the better topsoil is removed during construction of buildings and the alkaline subsoil is left behind.
     Chlorosis also occurs in compacted, poorly drained soils, and in landscapes where automatic sprinklers run daily or every other day. The low levels of oxygen and high levels of carbon dioxide that occur in these water-saturated soils interfere with iron uptake. Lawns heavily fertilized with nitrate and phosphorus may have chlorotic trees as well because an excess of these fertilizers interferes with micronutrient uptake.
     Correcting chlorosis begins with good cultural practices: water trees thoroughly but infrequently to allow the soil time to drain and gas exchange to occur; apply woodchip mulches below the canopy of the tree to encourage root growth and soil microbes that aid in nutrient uptake; and use fertilizers sparingly, particularly if the lawn is already being fertilized. Please note: driving nails into trees or burying them in the ground will NOT correct chlorosis
     If a soil test indicates a high pH, consider an application of granular sulfur to lower the pH, which improves micronutrient solubility and root uptake. One method involves core-aerating the area below the canopy of the tree, applying a high rate of sulfur (1 to 4 pounds or more for a 10x10 foot area) and watering it in well to prevent turf burn.
     Lowering the pH over a large area may be very difficult to do, especially on clay soils that resist changes in pH. In these situations, small pockets of soil can be altered by combining sulfur with iron sulfate or a micronutrient mix and placing it in holes dug into the ground around the tree.
    In areas where soil treatments may be ineffective or impractical, trunk injections may be used. Holes are drilled into the trunk flare, and a liquid micronutrient product containing either iron or manganese is injected into the holes; or the product is delivered to the tree through a needle-like injection tool.  Trunk injections are damaging to the tree and should be performed by a certified arborist who is well-trained in the procedure.
     If you are planning to plant new trees, choose ones that are less likely to develop chlorosis such as linden, elm, hackberry, most white oaks, ginkgo and Kentucky coffeetree.
     For more information, see the Nebraska Forest Service publication “Chlorosis of Trees in Eastern Nebraska."